Atlantic salmon in sea cages are held at densities many times higher than wild fish; crowding causes chronic stress, aggression, and fin damage that represent significant welfare problems.
Crowding in salmon aquaculture imposes multiple simultaneous welfare costs: reduced swimming space, increased agonistic interactions, elevated cortisol from chronic social stress, and elevated lice burden from close proximity. These stressors are not independent — crowding stress suppresses immune function that would otherwise limit lice infestation. Welfare certification maximum density standards represent an important tool, but enforcement and verification remain inconsistent.